Recent advances in wireless communication technology have enabled mobile terminals, such as cellular phones, personal digital assistants (PDA), and the like, to achieve greater signal quality and higher data transmission rates. These advances can be attributed, in part, to the complex data modulations schemes used in modern wireless communications circuits, such as GSM, EDGE, WCDMA, CDMA, IS-95, and so on. The modulation schemes typically produce a complex RF signal that is both phase modulated and amplitude modulated, generally referred to as non-constant envelope modulation. One way to realize/implement a transmitter for such a modulation scheme is to use polar modulation. An example of a polar modulation circuit may be found in published US application US20020077066 and published PCT application WO0237666, which are hereby incorporated by reference.
In polar modulation schemes such as the one described in US20020077066, a phase modulated RF signal having a constant amplitude is transmitted at varying power levels to realize the amplitude modulation. Varying the transmission power levels of the RF signal is performed by controlling a power amplifier in the mobile terminals. To maximize efficiency, the power amplifier is operated at or very near its highest available output level (i.e., in saturated mode), which is usually equal to the power supply level. The power supply is then varied to effect the amplitude modulation.
FIG. 1 illustrates the type of polar modulation circuit 100 described in US20020077066. As can be seen, the polar modulation circuit 100 includes a power amplifier 102 for amplifying an RF signal. The power amplifier 102 is configured to receive an RF input signal 104 and to output an RF output signal 106. Also present are a bias voltage 108 and a power supply 110 for providing power to the power amplifier 102. The power supply 110 is connected to the power amplifier 102 by an envelope controller 112 composed of transistors T1 and T2 and resistors Rg, R1, and R2, all connected as shown. The transistor T1 may be, for example, a bipolar junction transistor (BJT) and transistor T2 may be, for example, a metal-oxide semiconductor field-effect transistor (MOSFET). An amplitude modulation voltage 114 controls the envelope controller 112 to thereby modulate the power provided by the power supply 110 to the power amplifier 102.